Protocol | Interior or Exterior | Type | Hierarchy Required? | Metric |
OSPF | Interior | Link-state | Yes | Cost |
IS-IS | Interior | Link-state | Yes | Metric |
EIGRP | Interior | Advanced distance-vector | No | Composite |
BGP | Exterior | Path-Vector | No | Path-vector (attributes) |
When to Use BGP
BGP use in an AS is most appropriate when the effects of BGP are well understood and at least one of the following conditions exists:
- The AS allows packets to transit through it to reach other autonomous systems.
- He AS has multiple connections to other autonomous systems.
- The flow of traffic entering and leaving the AS must be manipulated.
When Not to Use BGP
Do not use BGP is one of the following conditions exists:
- A single connection to the Internet or another AS.
- Lack of memory or processor power on routers to handle constant BGP updates.
- You have limited understanding of route filtering and the BGP path-selection process.
- Low bandwidth between autonomous systems.
BGP Terminologies and Concepts
BGP Neighbor Relationships
General
· Any router that suns BGP is called a BGP speaker.
· A BGP peer must be configured under the BGP process with a neighbor command
External
· When BGP is running between routers in different autonomous systems, it is called External BGP (EBGP).
· An EBGP neighbor is outside this AS; an IGP is not run between the EBGP neighbors.
Internal
· When BGP is running between routers within the same autonomous systems, it is called Internal BGP (IBGP).
· Because multiple paths generally exist within an AS to reach other routers, a loopback address is usually used in the BGP neighbor command to establish the IBGP sessions.
· Routers running IBGP do not have to be directly connected to each other, as long as they can reach each other so that TCP handshaking can be performed to set up the BGP speakers’ relationship.
BGP Tables
The table of BGP information is known by the following names:
· BGP table
· BGP topology table
· BGP topology database
· BGP routing table
· BGP forwarding database
BGP Message Types
Open – After a TCP connection is established, the open message is sent by each side. The open message includes the following information:
· Version – 8-bit field indicating the messages BGP version number.
· My autonomous system – 16-bit field indicating the sender’s AS number.
· Hold Time – 16-bit field indicating the maximum number of seconds that might elapse between the receipt of successive keepalive or update messages by the sender.
· BGP router identifier (router ID) – 32-bit field indicates the sender’s BGP identifier.
· Optional parameters – A length field indicating the total length of the optional parameters field in octets. (currently only authentication is defined).
Keepalive – Keepalive messages are exchanged between peers often enough to keep the hold timer from expiring. If the negotiated hold time interval is 0, periodic keepalive messages are not sent.
Update – An update message has information on one path only; multiple paths require multiple messages. All the attributes in the message refer to that path, and the networks are those that can be reached through it.
An update message might include the following fields:
An update message might include the following fields:
· Withdrawn routes – A list of IP addresses prefixes for routes that are being withdrawn from service, if any.
· Path attributes – The AS-path, origin, local preference. Each path attribute includes the attribute type, attribute length, and attribute value. The attribute type consists of the attribute flags, followed by the attribute-type code.
· Network layer reachability information – This field contains a list of IP address prefixes that can be reached by this path.
Notification –A notification message is sent when an error condition is detected. The BGP connection is closed immediately after this is sent. Notification messages include an error code, an error subcode, and data related to the error.
BGP Attributes
Well-known Mandatory
A well-known attribute is one that all BGP implementations must recognize. These attributes are propagated to BGP neighbors. A well-known mandatory attribute must appear in a route’s description.
AS-Path (Type Code 2) – A list of AS numbers that a route has traversed to reach a destination, with the number of the AS that originated the route at the end of the list.
Next-Hop (Type Code 3) – Indicates the next-hop IP address that is to be used to reach a destination. The next hop address for a network from another AS is an IP address of the entry point of the next AS along the path to that destination network.
- For EBGP, the next hop is the IP address of the neighbor that sent the update.
- For IBGP, the protocol states that the next hop advertised by EBGP should be carried into IBGP.
Origin (Type Code 1) – Defines the origin of the path’s information. It can be one of three values.
- IGP (Value 0) – The route is interior to the originating AS. An origin on IGP is indicated with an i in the BGP table.
- EGP (Value 1) – The route is learned via EGP. This is indicated with an e in the BGP table.
- Incomplete (Value 2) – The route’s origin is unknown or is learned via some other means. This usually occurs when a route is redistributed into BGP.
Well-known Discretionary
A well-known attribute is one that all BGP implementations must recognize. A well-know discretionary attribute does not need to appear in a route description.
Local Preference (Type Code 5) – Indicates to routers in the AS which path is preferred to exit the AS. A path with a higher local preference is preferred. Local preference is an attribute that is configures on a router and
exchanges only among routers within the same AS. The default value for local preference on a
Cisco router is 100.
exchanges only among routers within the same AS. The default value for local preference on a
Cisco router is 100.
Atomic Aggregate (Type Code 6) - Indicates that the route has been summarized (aggregated) and that the AS Path may not contain an entire lis t of the Transit AS. It is used by a BGP speaker to inform other BGP speakers that the local system selected a less specific route without selecting a more specific route which is included in it.Optional Transitive
An optional attribute does not need to be supported by all BGP implementations; it could be a private attribute. An optional transitive attribute that is not implemented in a router should be passed to other BGP routers untouched.
Community (Type Code 8 (Cisco Defined)) – Used for destination (routes) that share common propertied and, therefore, share common policies; thus, routers act on the community rather than on individual routes. BGP communities are one way to filter incoming or outgoing routes. BGP communities allow routers to tag routes with an indicator (the community) and allow other routers to make decisions based on that tag.
Community (Type Code 7) - Contains the last AS number that formed the aggregate route (encoded as 2 octets), followed by the IP address of the BGP speaker that formed the aggregate route (encoded as 4 octets). Optional Non-Transitive
Multiexit-discriminator (MED) (Type Code 4) – Also called the metric in the Cisco IOS. It indicates to external neighbors the preferred path into an AS. This is a dynamic way for an AS to try to influence another AS as to which way it should choose to reach a certain route if there are multiple entrypoints into an AS. The lower metric value is preferred. MED is exchanged between AS’s and by default the MED comparison is done only if the neighboring AS is the same for all routes considered. Cisco Only
Weight – A Cisco defined attribute used for the path-selection process. The weight is configured locally to a router on a per-neighbor basis. This attribute provides local routing policies and is not propagated to any BGP neighbors. Routes with a higher weight are preferred when multiple routes to the same destination exist..BGP Operation
The BGP split horizon rule governs IBGP behavior. This BGP rule specifies that routes learned via IBGP are never propagated to other IBGP peers.
IGP Adjacencies
Interior routing protocols form adjacency relationships with directly connected neighbors.
IBGP Full Mesh
By fully meshing all IBGP neighbors, when a change is received from an external AS, the BGP router for this AS is responsible for informing all of its IBGP neighbors of the change. IBGP neighbors that receive this update do not send it to any other IBGP neighbor, because they assume the sending IBGP neighbor is fully meshed with all other IBGP speakers and has sent each IBGP neighbor the update.
BGP Synchronization
The BGP synchronization rule states that a BGP router should not use or advertise to an external neighbor a route learned by IBGP, unless that route is local or is learned from the IGP. If your AS is passing traffic from one AS to another, BGP should not advertise a route before all routers in your AS have learned about the router via IGP. BGP synchronization is on by default in current IOS releases.
Disabling BGP Synchronization
Before a BGP process can place networks it learned through an IBGP neighbor in the IP routing table, the route must be in the local routing table. The BGP and IGP must be synchronized before the networks learned from and IBGP neighbor can be used. If you disable synchronization, BGP can use networks learned from an IBGP neighbor that are not
present in the local routing table. Use synchronization id some routers in the BGO transit path in the AS are not
running BGP. Synchronization is on by default. Use the no synchronization command to disable it.
present in the local routing table. Use synchronization id some routers in the BGO transit path in the AS are not
running BGP. Synchronization is on by default. Use the no synchronization command to disable it.
Route Selection Decision Process
BGP chooses only a single best path to reach a specific destination
- (WEIGHT) Prefer the route with the highest weight.
- (LOCAL PREFERENCE) If the multiple routes have the same weights, prefer the route with the highest local preference.
- If the multiple routes have the same local preference, prefer the route that was originated by the local router.
- (AS-PATH) If none of the routes were originated by the local router, prefer the route with the shortest AS-path.
- (ORIGIN) If the AS-path length is the same, prefer the lowest-origin code.
- (MED) If all origin codes are the same, prefer the path with the lowest MED.
- (EBGP) If the routes have the same MED, prefer external paths (EBGP) over internal paths (IBGP).
- (IGP NEIGHBOR) If synchronization is disabled and only internal paths remain, prefer the path through the
- closest IGP neighbor. This means that the router prefers the shortest path within the AS to reach the destination.
- (OLDEST ROUTE) For EBGP paths, select the oldest route to minimize the effect of routes flapping.
- (ROUTER ID) Prefer the route with the lowest neighbor BGP router-ID value.
- (LOWEST NEIGHBOR IP) If the BGP router IDs are the same; prefer the route with the lowest neighbor IP address.
BGP Peer Groups
A BGP peer group is a group of BGP neighbors of the router being configured that all have the same update policies.
Peer groups simplify configurations and make them easier to read. They are more efficient because updates are
generated only once per peer group, rather than once per neighbor. A neighbor can only be part of one peer group.
Peer groups simplify configurations and make them easier to read. They are more efficient because updates are
generated only once per peer group, rather than once per neighbor. A neighbor can only be part of one peer group.
EBGP Multihop
Used when to EBGP speakers cannot be directly connected. Its configuration must include static routes or must enable
an IGP so that the neighbors can reach each other. If you have multiple physical connections between EBGP neighbors,
using a loopback interface and static routes to the loopback interface allows you to load balance the traffic between the multiple connections.
an IGP so that the neighbors can reach each other. If you have multiple physical connections between EBGP neighbors,
using a loopback interface and static routes to the loopback interface allows you to load balance the traffic between the multiple connections.
BGP Neighbor States
Idle – Indicated that the router does not know how to reach the IP addresses listed in the neighbor statement.
The router is idle for one of the following reasons:
The router is idle for one of the following reasons:
- It is waiting for a static route to that IP address or network to be configured.
- It is waiting for the local routing protocol (IGP) to learn about this network through an advertisement from
- another router.
Active – Indicated that is has found the IP address in the neighbor statement and has created and sent out a BGP open packet but has not received a response (open confirm packet) back from the neighbor.
Established – Indicates that both routers agree to exchange BGP updates with one another and routing has begun.
Facts
1. BGP is an Interdomain Routing Protocol (IDRP), which is also known as an EGP.
2. BGP version 4 (BGP-4) is the latest version of BGP.
3. The classic definition of an AS is “a set of routers under a single technical administration, using an Interior
Gateway Protocol and common metrics to route packets within an AS, and using an Exterior Gateway Protocol to route packets to other [autonomous systems]”
Gateway Protocol and common metrics to route packets within an AS, and using an Exterior Gateway Protocol to route packets to other [autonomous systems]”
4. The Internet Assigned Numbers Authority (IANA) is responsible for allocating AS numbers.
5. The AS designator is a 16-bit number with a range of 1 to 65535.
6. The range of AS numbers 64512 to 65535 is reserved for private use.
7. The main goal of BGP is to provide an interdomain routing system that guarantees the loop-free exchange of routing information between autonomous systems.
8. BGP routers exchange information about paths to destination networks.
9. BGP-4 and its extensions are the only acceptable version of BGP available for use on the public-based internet.
10. BGP is also a distance-vector protocol, with many enhancements. It is also called a path-vector protocol
11. BGP specifies that a BGP router can advertise to its peers in the neighboring autonomous systems only those routes that it uses.
12. You cannot influence how the neighbor AS will route your traffic, but you can influence how your traffic gets to a neighbor AS.
13. BGP uses TCP as its transport protocol, which provides connection-oriented reliable delivery.
14. BGP information is carried inside TCP segments using protocol 179; these segments are carried inside IP packets.
15. BGP is the only IP routing protocol to use TCP as its transport layer.
16. Two routers speaking BGP establish a TCP connection with one another and exchange message to open and confirm the connection parameters. These two routers are called BGP peer routers or BGP neighbors.
17. TCP uses a dynamic window, which allows for 65,576 bytes to be outstanding before it stops and waits for an acknowledgement.
18. The BGP AS patch is guaranteed to always be loop-free. A router running BGP does not accept a routing update
that already includes its AS number in the path list, because the update has already passed through its AS,
and accepting it again will result in a routing loop.
that already includes its AS number in the path list, because the update has already passed through its AS,
and accepting it again will result in a routing loop.
19. Any router that suns BGP is called a BGP speaker.
20. A BGP peer must be configured under the BGP process with a neighbor command.
21. Because multiple paths generally exist within an AS to reach other routers, a loopback address is usually used
in the BGP neighbor command to establish the IBGP sessions.
in the BGP neighbor command to establish the IBGP sessions.
22. EBGP router (BGP routers learned from an external AS) have an AD of 20. IBGP routes (VGP routes learned from within the AS) have an AD of 200.
23. BGP does not use any transport protocol-based keepalive mechanism to determine whether peers can be reached. Instead, keepalive messages are exchanged between peers often enough to keep the hold timer from expiring.
If the negotiated hold time interval is 0, periodic keepalive messages are not sent.
If the negotiated hold time interval is 0, periodic keepalive messages are not sent.
24. A notification message is sent when an error condition is detected. The BGP connection is closed immediately after this is sent. Notification messages include an error code, an error subcode, and data related to the error.
25. Keepalive messages consist of only a message header and have a length of 19 bytes; they are sent every 60 seconds by default. Other messages might be between 10 and 4096 bytes long. The default hold time is 180 seconds.
26. When running BGP over a multiaccess network such as Ethernet, a BGP router uses the appropriate address as the next-hop address (by changing the next-hop attribute) to avoid inserting additional hops in the network.
This feature is sometimes called a third-party next hop.
This feature is sometimes called a third-party next hop.
27. Because TCP cannot use broadcasting, BGP cannot use it either, so BGP has to fully mesh TCP sessions.
28. Most modern transit autonomous systems run full-mesh IBGP and disable synchronization.
29. BGP does not accept unsolicited updates; it must be aware of every neighboring router and have a neighbor statement for it.
30. EBGP assumes a TTL of 1.
31. If you have multiple physical connections between EBGP neighbors, using a loopback interface and static
routes to the loopback interface allows you to load-balance the traffic between the multiple connections.
routes to the loopback interface allows you to load-balance the traffic between the multiple connections.
32. The neighbor command tells BGP where to advertise. The network command tells BGP what to advertise.
33. Before a BGP process can place networks it learned through an IBGP neighbor in the IP routing table,
the route must be in the local routing table.
the route must be in the local routing table.
34. If you disable synchronization, BGP can use networks learned from an IBGP neighbor that are not present in
the local routing table.
the local routing table.
35. Use synchronization if some routers in the BGP transit path in the AS are not running BGP (and, therefore, do not have a full-mesh IBGP).
36. Synchronization is on by default.
Verifying and Troubleshooting BGP
show ip bgp [ip-address [mask [longer-prefixes [injected] | shorter-prefixes [length]]] | all | oer-paths | prefix-list name | route-map name] – Displays entries in the BGP topology database (BGP table). Specify a a network number to get more specific information about a particular network. |
BGP table version is 21, local router ID is 10.100.64.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.0.0.0 10.100.64.2 0 65005 65008 ? *> 10.1.1.0/24 10.100.64.2 0 65005 65008 65003 i *> 10.2.1.0/24 10.100.64.2 0 65005 65008 65003 i *> 10.4.0.0/16 0.0.0.0 0 32768 ? *> 10.5.1.0/24 10.100.64.2 0 0 65005 ? *> 10.100.0.0/24 10.100.64.2 0 65005 65008 65003 i *> 10.100.32.0/24 10.100.64.2 0 65005 65008 i *> 10.100.48.0/24 10.100.64.2 0 0 65005 ? *> 10.100.64.0/24 0.0.0.0 0 32768 ? * 10.100.64.2 0 0 65005 ? *> 10.100.80.0/24 10.100.64.2 0 0 65005 ? *> 192.168.1.0 10.100.64.2 0 65005 65008 ? *> 192.168.1.64/28 10.100.64.2 0 65005 65008 65003 i *> 192.168.1.112/28 10.100.64.2 0 65005 65007 65006 i |
· The 1st Column
o An s indicates that route summarization has been done and that the specified routes are suppressed.
o A d, for dampening, indicates that the route is being dampened (penalized) for going up and
down too often.
down too often.
o An h, for history, indicates that the route is unavailable and probably down; historic information
about the route exists, but a best route does not exist.
about the route exists, but a best route does not exist.
· The 2nd Column – A greater-than sign (>) indicates the best path for a route selected by BGP; this route is
offered to the IP routing table.
offered to the IP routing table.
· The 3rd column is either blank or it has an i in it. If it is blank, BGP learned that route from an external peer.
If it has an i, an IBGP neighbor advertised this route to this router.
If it has an i, an IBGP neighbor advertised this route to this router.
· The 4th column lists the networks that the router learned.
· The 5th column lists all the next-hop addresses for each route. The next-hop address column might contain 0.0.0.0, which signifies that this router originated the route.
· The 6th, 7th, 8th columns list three BGP path attributes associated with the path (MED, Local Preference and Weight).
· The 9th column contains the AS path information. The first AS listed is the adjacent AS that this network was learned from. The last number is this network’s originating AS.
· The 10th column signifies how the route was entered into BGP on the original router.
o i - the original router probably used the network command to introduce
o e – signifies that the original router learned this network from EGP
o ? – signifies that the original BGP process cannot absolutely verify this network’s availability,
because it is redistributed from an IGP into the BGP process.
because it is redistributed from an IGP into the BGP process.
show ip bgp neighbors [all] [ip-address [advertised-routes | dampened-routes | paths [regexp] | received prefix-filter | received-routes | routes]] – Displays detailed information about the TCP and BGP connection to neighbors. |
p7r1#sh ip bgp neighborsBGP neighbor is 10.7.7.100, remote AS 65200, external link BGP version 4, remote router ID 172.16.13.100 BGP state = Established, up for 00:24:08 Last read 00:00:08, hold time is 180, keepalive interval is 60 seconds Neighbor capabilities: Route refresh: advertised and received Address family IPv4 Unicast: advertised and received Received 29 messages, 0 notifications, 0 in queue Sent 28 messages, 0 notifications, 0 in queue Route refresh request: received 0, sent 0 Default minimum time between advertisement runs is 30 seconds For address family: IPv4 Unicast BGP table version 5, neighbor version 5 Index 1, Offset 0, Mask 0x2 3 accepted prefixes consume 108 bytes Prefix advertised 1, suppressed 0, withdrawn 0 Connections established 1; dropped 0 Last reset neverConnection state is ESTAB, I/O status: 1, unread input bytes: 0Local host: 10.7.7.7, Local port: 11005Foreign host: 10.7.7.100, Foreign port: 179 |
show ip bgp summary – Displays the status on all BGP connections. |
BGP Table Version – Increments when the BGP table changes
Neighbor – The IP address used in the neighbor statement with which this router is setting up a relationship
Version (V) – The version of BGP this router is running with the listed neighbor.
AS – The neighbor’s AS number.
Messages received (MsgRcvd) – The number of BGP messages received from this neighbor.
Messages Sent (MsgSent) – The number of BGP messages sent to this neighbor.
TblVer – The last version on the BGP table that was sent to that neighbor.
In queue (InQ) – The number of messages from this neighbor that are waiting to be processed.
Out queue (OutQ) – The number of messages queued up and waiting to be sent to this neighbor.
TCP flow prevents this router from overwhelming a neighbor with a large update.
TCP flow prevents this router from overwhelming a neighbor with a large update.
Up/Down – The length of time this neighbor has been in the current BGP state (established, active, or idle).
State – The current state of the BGP session – active, idle, open sent, open confirm, or idle (admin).
Prefix received (PfxRcd) – When the session is in the established state, this number represents how many
BGP network entries have been received from this neighbor.
BGP network entries have been received from this neighbor.
debug ip bgp updates [access-list | expanded-access-list] [in | out] [events] |
Acronyms
IGP | Interior Gateway Protocol |
EGP | Exterior Gateway Protocol |
IDRP | Interdomain Routing Protocol |
IANA | Internet Assigned Numbers Authority |
AS | Autonomous System |
TCP | Transmission Control Protocol |
MED | Multiexit Discriminator |
IBGP | Interior Border Gateway Protocol |
EBGP | Exterior Border Gateway Protocol |
BGP | Border Gateway Protocol |
Configuration Commands
ip route prefix mask {address | interface} [distance] [permanent] [tag tag] | A global configuration command that defines a static route entry in the IP routing table. |
bgp always-compare-med | Forces the comparison of the BGP MED attribute to be done, even if the neighboring AS is not the same for all the routes considered |
bgp bestpath missing-as-worst | Forces BGP routes without the MED attribute to have a MED value of infinity, making the route least preferred. |
bgp cluster-id cluster-id | Configures the cluster ID; used if a BGP cluster has not than one route reflector. |
bgp default local-preference number | Defines the default BGP local-preference attribute value |
neighbor {ip-address | peer-group-name} distribute-list {access-list-number | expanded-list-number | access-list-name | prefix-list-name} {in | out} | Distributes BGP neighbor information; as specified in an access list |
neighbor ip-address | peer-group-name ebgp-multihop [ttl] | Allows the router to accept and attempt BGP connections to external peers residing on the networks that are not directly connected. |
neighbor peer-group-name peer-group | Creates a neighbor peer group. |
neighbor ip-address peer-group peer-group-name | Assigns neighbors as part of a peer group |
neighbor {ip-address | peer-group-name} prefix-list {prefix-list-name | clns-filter-expr-name | clns-filter-set-name} {in | out} | Identifies a prefix list to be used to filter BGP routes from or to a peer router. |
neighbor {ip-address | peer-group-name} remote-as as-number | Identifies a peer route with which this router will establish a BGP session. |
neighbor {ip-address | peer-group-name} route-map map-name {in | out} | Applies a route map to incoming or outgoing BGP routes. |
neighbor ip-address route-reflector-client | Defines the router as a BGP route reflector and identifies the specified neighbor as its route reflector client. |
neighbor {ip-address | peer-group-name} send-community [both | standard | extended] | Specifies that the BGP communities attribute should be sent to a BGP neighbor. |
neighbor {ip-address | peer-group-name} shutdown | Disables an existing BGP neighbor or neighbor peer group. |
neighbor {ip-address | peer-group-name}soft-reconfiguration [inbound] | Forces the router ro store all received BGP updates without modifications so that they can he used to do an inbound soft reconfiguration. |
neighbor ip-address | peer-group-name weightnumber | Defines the BGP weight attribute to a neighbor connection. |
set as-path {tag | prepend as-path-string} | Modifies the AS path for BGP routes |
set automatic-tag | Automatically computes the BGP tag value. |
set community {community-number [additive] [well-known-community] | none} | Sets the BGP communities attribute within a route map. |
set local-preference | Specifies a BGP local preference value for the AS path from a route map |
set metric metric-value | Sets the BGP metric (MED) value from a route map |
set origin {igp | egp as-number | incomplete} | Specifies the BGP origin code. |
set weight number | Specifies the BGP weight for the routing table. |
Show Commands
show ip bgp [ip-address [mask [longer-prefixes [injected] | shorter-prefixes [length]]] | all | oer-paths | prefix-list name | route-map name | Displays the BGP routing table; specify a network number to get more specific information about a particular network. |
show ip bgp neighbors [all] [ip-address [advertised-routes | dampened-routes | paths [regexp] | received prefix-filter | received-routes | routes]] | Displays information about the TCP and BGP connections to neighbors |
sh ip bgp summary | Displays the status of BGP connections. |
Debug Commands
debug ip bgp [A.B.C.D. | dampening | events |in | keepalives | out | updates | vpnv4 | mpls] | Displays information related to processing of the Border Gateway Protocol (BGP), |
debug ip bgp groups [index-group | ip-address] | Displays information related to the processing of Border Gateway Protocol (BGP) update-groups. |
debug ip bgp updates [access-list | expanded-access-list] [in | out] [events] | Displays information about the processing of Border Gateway Protocol (BGP) updates |
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